A vehicle such as an automobile is equipped with a stabilizer (a stabilizer bar or an anti-roll bar) for suppressing roll of a vehicle body due to vertical movement of wheels. The stabilizer generally includes a torsion portion extending in a vehicle width direction and a pair of left and right arms bent and formed in a vehicle front-rear direction, and is formed of a substantially U-shaped rod body. In the vehicle, the distal ends of the arms are respectively connected to suspension devices of the wheels, and the torsion portion is inserted into bushes fixed to the vehicle body, so that the stabilizer is suspended between left and right suspension devices and is supported by the bushes.
When the vehicle turns or rides over undulation of a road surface during driving, a stroke difference occurs between the left and right suspension devices due to vertical movement of left and right wheels. At this time, loads (displacements) due to the stroke difference between the suspension devices are respectively input to the arms of the stabilizer, and the torsion portion is twisted by the loads (a displacement difference) from the arms, so that an elastic force to restore torsional deformation is generated. The stabilizer suppresses a vertical displacement difference between the left and right wheels by the elastic force to restore the torsional deformation, and increases roll rigidity of the vehicle body, so that the roll of the vehicle body is suppressed.
As a form of the stabilizer, there are a solid stabilizer having a solid structure and a hollow stabilizer having a hollow structure. The solid stabilizer is characterized by excellent mechanical strength and low manufacturing cost. In contrast, the hollow stabilizer is not easy to secure the mechanical strength as compared with the solid stabilizer, but is a form suitable for reducing weight of the vehicle. As a material of the hollow stabilizer, an electroseamed steel pipe, a seamless steel pipe, a butt-welded steel pipe or the like is generally used. Among them, the electroseamed steel pipe is often used as the material of the hollow stabilizer because it is low in manufacturing cost and excellent in mass productivity.
The hollow stabilizer is often manufactured by subjecting such a steel pipe to a bending process to form a product shape, and by subjecting it to heat treatment. As a bending process, for example, cold bending using an NC bender or hot bending using a total bending die is performed depending on thickness and diameter of the steel pipe. In general, oil quenching or water quenching and tempering are performed as the heat treatment. Or, a steel pipe subjected to cold bending may be subjected to an as-roll type process in which annealing is performed instead of quenching and tempering. Then, the steel pipe subjected to the heat treatment is commercialized through a finishing process such as a surface treatment process by shot peening, or a coating process.
In recent years, in vehicles such as an automobile, the weight of the vehicle tends to be heavy due to, for example, mounting of an electric motor or a secondary battery. Along with this, in order to withstand higher stress, further improvement of the mechanical strength, fatigue durability or the like is also required for the hollow stabilizer. Conventionally, as the material of the hollow stabilizer, a thin-walled electroseamed pipe, in which a ratio (t/D) of a thickness (t) to an outer diameter (D) of the pipe is relatively small, the thickness of the pipe is less than about 5.5 mm, and dimensional accuracy and formability are good, has been used. However, at present, thicker steel pipes are manufactured for a wide variety of outer diameters by hot diameter reduction rolling of the electroseamed pipe having a large diameter and a thick wall. Since a selection range of the ratio (t/D) of the thickness (t) to the outer diameter (D) of the steel pipe is extended, design range of the hollow stabilizer focusing on securing the mechanical strength is also extended.
For example, as a technology for providing the steel pipe of low cost and good quality, Patent Document 1 discloses an electroseamed welded steel pipe for the hollow stabilizer in which the ratio t/D of the thickness t to the outer diameter D is 20% or more. Further, Patent Document 1 describes that the electroseamed welded steel pipe for the hollow stabilizer, in which the ratio t/D of the thickness t to the outer diameter D is 20% or more, can be obtained by employing the electroseamed welded steel pipe obtained by diameter reduction rolling after electric resistance welding, and by hot reduction rolling of the steel pipe by a stretch reducer, it is possible to reduce the outer diameter of the steel pipe, resulting in increasing the t/D as compared to before the reduction rolling.
Further, as a technology for obtaining durability of the stabilizer, Patent Document 2 discloses a method of manufacturing the hollow stabilizer, including a pipe compressing step of compressing an electroseamed pipe in a temperature range of a hot state or a warm state so as to make a ratio of a thickness to an outer diameter 18 to 35%, a forming step of forming the compressed electroseamed pipe into a stabilizer shape in a cold state, a step of applying heat treatment to a formed half-finished stabilizer, a shot peening step of impacting shot on the half-finished stabilizer, and a step of coating the half-finished stabilizer.